1. Field of the Invention
The present invention relates to an exhaust control system provided with an exhaust control valve for controlling the opening/closing timing of an exhaust port in accordance with an operating condition of a two-cycle internal combustion engine.
2. Description of Background Art
In a conventional two-cycle internal combustion engine an exhaust control system is provided which has an exhaust control valve which changes the opening/closing timing of an exhaust port for preventing blow-by of fresh gas while the engine is operating under a condition where there is a small intake volume; for example, when the engine is operating at a low revolution. In addition, the opening/closing timing of the exhaust port has been controlled to ensure a high output while the engine is operating under a condition where there is a large intake volume; for example, when the engine is operating at a high revolution. An example of such an exhaust control valve will be described below with reference to FIG. 6.
This exhaust control valve, indicated at a, is swingably supported by a cylinder e, the cylinder e being formed with an exhaust passage having a pair of exhaust ports c and d which are open to a peripheral wall surface of a cylinder bore b with a piston fitted reciprocatably therein. The pair of exhaust ports c and d are positioned on both sides in the circumferential direction of a reinforcing wall f which extends axially on the circumferential wall of the cylinder bore b. The exhaust control valve a has a pair of valve elements g and h which are disposed correspondingly to the exhaust ports c and d.
At a predetermined number of revolutions in a low speed region of the internal combustion engine, the pair of valve elements g and h assume respective first positions indicated with solid lines in FIG. 6, in which control surfaces k and m of the valve elements g and h close upper port portions of the exhaust ports c and d so that an opening timing of the exhaust ports c and d is made at a latest point and a closing timing thereof is made at an earliest point, to suppress blow-by of fresh gas when the engine is at a small intake volume and improve the combustion stability. On the other hand, at a predetermined number of revolutions in a high speed region of the internal combustion engine, the control surfaces k and m of the valve elements g and h assume respective second positions indicated with dash-double dot lines in FIG. 6 and the control surfaces k and m open the upper port portions, so that the opening timing of the exhaust ports c and d is made at an earliest point and the closing timing thereof is made at a latest point, whereby the exchange of gases between burned gas and fresh gas by exhaust and scavenging when the engine is operating at a large intake volume is performed smoothly and positively and there is obtained a high engine output.
Lower edge portions n and p of the control surfaces k and m are formed in a shape that substantially coincides with the shape of upper edge portions of the exhaust ports c and d when the valve elements g and h assume their second positions. Therefore, when the valve elements g and h assume their first positions, the lower edge portions n and p of the control surfaces k and m are inclined downward from portions close to the reinforcing wall f to circumferential ends thereof. The reason why the lower edge portions n and p are formed so as to coincide with the shape of the upper edge portions of the exhaust ports c and d at the second positions of the valve elements g and h is that if the lower edge portions n and p of the control surfaces k and m are positioned above the upper edge portions of the exhaust ports c and d and a dead space is formed therebetween, output characteristics in a high speed region of the engine are deteriorated and a desired high output cannot be obtained. Although this has been made sure by experiment, the cause is presumed to be because of the fact that a smooth exhaust of burned gas does not occur under the influence of vortices created in the dead space.
When the piston moves down to a position lower than the center of the lower edge portions n and p, the exhaust ports c and d begin to be opened by the piston, the valve elements g and h being provided with the control surfaces k and m which include the lower edge portions n and p. In this state, circumferential ends of the exhaust ports c and d are still closed by the control surfaces k and m. With subsequent descent of the piston, the exhaust ports c and d are opened gradually from the center of the lower edge portions n and p toward end portions thereof.
Having made an experiment in connection with the above background art, the inventors have obtained such results as indicated with broken lines in FIG. 5. The graph of FIG. 5 illustrates changes in internal pressure of a combustion chamber during the lapse of time just after the start of opening of a throttle valve, which is in an idle opening condition in a two-cycle internal combustion engine provided with an exhaust control valve and assuming an idling state. In the same figure, in a period Txe2x80x2 which starts just after the start of opening of the throttle valve, the internal pressure of the combustion chamber varies largely at a variation width Hxe2x80x2 and it can be seen that unstable combustion involving alternate occurrence of misfire and combustion in the next cycle of gas left unburned by the misfire, i.e., irregular combustion, occurs over a relatively long period. The misfire is caused by retention of an excess of burned gas in the cylinder bore b. At least during this period Txe2x80x2, the number of revolutions of the internal combustion engine is smaller than a predetermined number of revolutions in the foregoing low speed region, so that the valve elements g and h of the exhaust control valve occupy their first positions. Thus, the reason why such irregular combustion occurs when the valve elements g and h of the exhaust control valve occupy their first positions is presumed to be because the exhaust ports c and d are opened later at circumferential ends than at the center thereof. This is attributable to the shape of the lower edge portions n and p of the control surfaces k and m, in the vicinity of the late opening portions in the cylinder bore b. Accordingly, exhaust and scavenging are not conducted to a satisfactory extent, resulting in excess burned gas remaining within the cylinder bore b, thus facilitating the occurrence of irregular combustion.
In order to improve the exhaust and scavenging functions, if downwardly extending portions from both circumferential ends of the control surfaces k and m are shortened to quicken the timing at which circumferential ends of the exhaust ports c and d are opened, the period of occurrence of irregular combustion decreases. However, when the valve elements g and h assume their second positions in the high speed region, the foregoing dead space occurs, thus making it impossible to obtain a desired high output of the internal combustion engine throughout the entire operating range.
The present invention has been accomplished in view of the above-mentioned circumstances and it is an object of the invention to provide an exhaust control system in a two-cycle internal combustion engine capable of suppressing the occurrence of irregular combustion when the opening timing of the exhaust ports is set to a delay timing including a maximum delay timing, to improve the combustion stability. Furthermore, it is an object of the present invention to afford a high engine output when the opening timing of exhaust ports is set to a maximum advance timing.
A first aspect of the present application resides in an exhaust control system in a two-cycle internal combustion engine which includes an exhaust control valve disposed in a cylinder thereof to control the opening/closing timing of an exhaust port. The cylinder is formed with an exhaust passage having an exhaust port, the exhaust port being formed in a peripheral wall surface of a cylinder bore and adapted to be opened and closed by a piston, the piston being fitted into the cylinder bore reciprocatably. Furthermore, the exhaust control valve has a valve element movable between a most projected position and a most retracted position within the exhaust passage, the valve element having a control surface which, when the valve element assumes the most projected position, faces the cylinder bore and closes an upper port portion of the exhaust port and which, when the valve element assumes the most retracted position, opens the upper port portion. A lower edge portion of the control surface has a top portion and a lower portion which respectively assume a top position and a position lower than the top position when the valve element occupies the maximum projected position, the opening timing of the exhaust port being set to a maximum delay timing when the valve element occupies the maximum projected position and being set to a maximum advance timing when the valve element occupies the maximum retracted position, wherein the lower edge portion of the control surface is formed in a shape which substantially coincides with the shape of an upper edge portion of the exhaust port when the valve element occupies the most retracted position. In addition, the valve element is formed with a communication passage, one end of which, when the valve element occupies the maximum projected position, opens to a lower control surface portion of the control surface positioned lower than the top portion and the other end of which opens to the exhaust passage directly.
According to the first aspect of the present application, during the period from the state in which the valve element of the exhaust control valve occupies the maximum projected position until when one end portion of the communication passage is fully closed by the cylinder formed with the upper edge portion of the exhaust port, and while the opening timing of the exhaust port is set to the delay timing including the maximum delay timing, the exhaust port is opened later than the opening timing of the exhaust port at the top of the lower edge portion by means of the piston which is in a descending stroke, at the lower portion of the lower edge portion located near the position where one end portion of the communication passage opens in the lower control surface portion. Consequently, burned gas present near the lower control surface portion in the cylinder bore flows out directly to the exhaust passage through the communication passage at a timing earlier than in the background art and in an amount larger by an amount corresponding to the flow in the communication passage, whereby the exhausting action near the lower control surface portion in the cylinder bore is improved, the scavenging action which follows the exhaust is also improved, the amount of residual burned gas is reduced, and the occurrence of irregular combustion is suppressed.
Even in the event of occurrence of irregular combustion, the amount of unburned gas (air-fuel mixture) remaining within the cylinder bore during misfire decreases by an amount corresponding to an outflow from the communication passage to the exhaust passage. Therefore, in the next cycle, the intake volume in the cylinder bore, which is the sum of the amount of remaining unburned gas and the amount of fresh gas newly introduced, becomes smaller than that in the background art and the combustion pressure becomes low. Accordingly, the variation range of the internal pressure in the combustion chamber upon occurrence of irregular combustion becomes narrower and vibrations caused by irregular combustion are diminished. When the valve element of the exhaust control valve assumes its retracted position and the opening timing of the exhaust port is set to the maximum advance timing, the lower edge portion of the control surface substantially coincides with the upper edge portion of the exhaust port, so that a dead space is not formed by the valve element and the exhaust port and the exhaust of burned gas is performed smoothly and positively by both exhaust and scavenging actions in the piston descending stroke.
As a result, when the opening timing of the exhaust port is set to the delay timing, including the maximum delay timing, by the exhaust control valve, the amount of residual gas in the vicinity of the lower control surface portion within the cylinder bore decreases, whereby the occurrence of irregular combustion is suppressed to improve the combustion stability and shorten the occurrence period of irregular combustion. Consequently, the total amount of unburned gas discharged by misfiring during the irregular combustion period decreases and hence the fuel economy and exhaust emission are improved. In addition, since the range of pressure variation in the combustion chamber upon occurrence of irregular combustion becomes narrower, vibrations attributable to irregular combustion are diminished. Furthermore, when the opening timing of the exhaust port is set to the maximum advance timing by the exhaust control valve, burned gas is discharged from the exhaust port smoothly and positively, so that a high engine output is obtained.
According to a second aspect of the present application, in addition to the first aspect, an exhaust control system in a two-cycle internal combustion engine includes a top portion positioned near a circumferential center of the exhaust port. The communication passage comprises a pair of communication passages which are a pair of through holes independent of each other. Furthermore, first ends of the communication passages are open, respectively, to positions near both peripheral end-side edges in the lower control surface portion located close to both circumferential end-side edges of the exhaust port.
According to the second aspect of the present application, there are obtained the following effects in addition to the effects of the first aspect of the present application. At the lower portion near both circumferential end-side edges in the lower control surface portion to which are open the first end portions of the paired communication passages constituted by a pair of mutually independent through holes, the exhaust port is opened later than the opening timing of the exhaust port at the top portion of the lower edge located near the center of the exhaust port, by means of the piston which is descending. Furthermore, both circumferential end-side edges are positioned near the circumferential side edge of the exhaust port. Therefore, burned gas present near the circumferential end-side edges where the gas exhibits a relatively small fluidity in the cylinder bore, flows out through the communication passages to the exhaust passage at an earlier timing than in the background art and in an amount larger by an amount corresponding to the flow in the communication passages. Accordingly, in a portion included in a wide circumferential range centered at the center of the exhaust port within the cylinder bore and where the fluidity of gas is relatively low, both exhaust and scavenging actions are improved and the amount of burned gas remaining within the cylinder bore decreases, whereby the occurrence of irregular combustion is suppressed effectively.
As a result, the amount of residual burned gas decreases in a portion included in a wide circumferential range centered at the center of the exhaust port within the cylinder bore and where the fluidity of gas is relatively low. Accordingly, in connection with the effects of the first aspect of the present application, the occurrence of irregular combustion is suppressed effectively, the combustion stability is improved, and the period of occurrence of irregular combustion is shortened.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.